Shipping containers at the Port of Los Angeles. Unions are stepping up their efforts to thwart White House plans for passing foreign trade deals on a "fast track" through Congress. Nick Ut/AP hide caption

itoggle caption Nick Ut/AP

Wisconsin Gov. Scott Walker became a Republican political star by taking on his state's public employee unions. This week he signed a bill that would weaken private-sector unions. Cliff Owen/AP hide caption

itoggle caption Cliff Owen/AP

Hundreds of union members rally outside the Capitol in Madison on Tuesday to oppose a Republican-led measure that would make Wisconsin a right-to-work state. Reuters/Landov hide caption

itoggle caption Reuters/Landov

Fast-food workers in Los Angeles march in August 2013 to raise the minimum wage to $15 an hour. Similar protests around the country have been organized by labor unions. Nick Ut/AP hide caption

itoggle caption Nick Ut/AP

Wisconsin Gov. Scott Walker spoke about taking on public employee unions, and the protests that followed, at a recent candidates forum in Iowa. He said what people may not know is that protesters — as many as 1,000 of them — showed up outside his home while his family was there. He says he also received death threats. Scott Olson/Getty Images hide caption

itoggle caption Scott Olson/Getty Images

Market Basket meat manager Bob Dietz of Methuen, Mass., (center) and other workers celebrate after watching a televised speech last month by restored Market Basket CEO Arthur T. Demoulas at a store in Chelsea. Steven Senne/AP hide caption

itoggle caption Steven Senne/AP

Nancy Becker, an Amazon employee in Bad Hersfeld, Germany, speaks at a protest rally outside the company's headquarters in Seattle in December. Ted S. Warren/AP hide caption

itoggle caption Ted S. Warren/AP

Members of the California Nurses Association say they rallied in Sacramento in May to raise public awareness of their concerns about patient care in California hospitals. April Dembosky/KQED hide caption

itoggle caption April Dembosky/KQED

Picketers supporting independent truck drivers at the ports of Los Angeles and Long Beach stand outside a container terminal. Kirk Siegler/NPR hide caption

itoggle caption Kirk Siegler/NPR